An old patent document filed by one of the present applicants, i.e., Patent Document 1: Japanese Unexamined Patent Publication (kokai) No. Hei 4-83914, proposes to apply on the surface of an aluminum-based bearing alloy a resin-based and solid-lubricant-containing coating. In that patent document, the following is described: “A solid lubricant or the like is diluted with an appropriate diluting agent. The solid lubricant, diluting agent and polyimide resin are applied on a lining by means of spraying. Drying and baking are carried out at 150 to 300 degrees C. Other methods such as tumbling, immersion and brushing are also possible. The coating thickness is preferably 1 to 25 μm.” That document is silent about particle diameter of the solid lubricant.
A typical solid lubricant has a crystal structure in which (001) planes are formed of atoms bonded in a network and are laminated parallel to one another. Parallel cleavage along the crystal planes provides a low friction property. Sliding members, which take an advantage of this property, are largely classified into the following types. In one type, a solid lubricant is directly applied on a substrate. In the other type, a solid lubricant dispersed in a resin binder is applied on the substrate to provide a coating layer. The former type of sliding member is disclosed in Patent Document 2: Japanese Unexamined Patent Publication (kokai) No. 2007-139149 and Patent Document 3: Japanese Unexamined Patent Publication (kokai) No. 2007-270205. The plate-form crystals of a solid lubricant are oriented in said one type of the orientation and the index of (001) plane of these crystals is as high as 90% or more. However, since only the solid lubricant is bonded to a substrate in said one type of sliding member, adhesion is weak and hence serious wear occurs. In addition, when one tries to form a coating layer only by using solid lubricant, it is transferred to the coating layer at low yield. Neither Patent Document 1 nor Patent Document 2 describes, in Detailed Explanation of Invention, the particle diameter of solid lubricant.
Meanwhile, the present applicants disclosed in Patent Document 4, Japanese Unexamined Patent Publication (kokai) No. 2008.95725 that the solid lubricant can exhibit a relative C-axis intensity ratio of 85% or more in the resin-based coating layer of said type. The relative C-axis intensity ratio=a percentage ratio of accumulative intensities of (002), (004) and (008) planes relative to the accumulative intensities of (002), (004), (100), (101), (102), (103), (105), (110) and (008) planes detected by X-ray diffraction. The (002), (004) and (008) planes among the nine planes mentioned are cleavage planes. With regard to the other planes, i.e. (004), (100), (101), (102), (103), (105), (110) and (008) planes, diffraction angle 20 can be obtained by the same method. Diffraction peaks from the planes other than the nine planes mentioned above may be obtained, but the intensity of the peaks is extremely low. The diffraction peaks of these other planes can therefore be neglected in the calculation of relative C-axis intensity ratio. Even when the orientation attained is “vertical”, since X ray necessarily deflects from the six planes other than the cleavage planes, the relative C-axis intensity ratio cannot arrive at 100%. The conditions described hereinbelow must be adjusted to make the orientation close to “vertical”. Nevertheless, at present, the highest attainable relative C-axis intensity ratio seems to be slightly higher than 90%.
Patent Document 4 recognizes that it is difficult to attain a relative C-axis intensity ratio of 90% or more. Patent Document 4 also describes a coating method which affects the orientation of a solid lubricant. Solid lubricant particles have such a property that they are liable to cleave in a direction parallel to the surface of the particles. This property is attributable to the crystal structure of solid lubricant. Therefore, the solid lubricant is rendered to have a thin plate form through a crushing process of raw material. Blended solid lubricant in thin-plate form and resin are subjected to spraying, brushing, rolling, dipping (immersion) and the like. The resultant coating is 2 to 10 μm, preferably 2 to 8 μm in thickness. A method for reducing the viscosity of paint is employed. Particle diameter of the solid lubricant is not particularly described. In such a method, main factors that hinder achievement of high orientation are high viscosity of a paint, mutual contact of the solid-lubricant particles and rapid vaporization of solvent. Therefore, it can be said that that patent document admits that, although these factors may be controlled to enhance orientation, a successful resin-based coating cannot be provided.
Patent Document 5, Japanese Unexamined Patent Publication (kokai) No. 2002-339083 is related to blasting material which contains approximately 95% by weight or more of molybdenum disulfide and has an average particle diameter of approximately 1 μm to approximately 20 μm. This material is blasted on, for example, the surface of a piston or the like to impart a low-frictional property. The specification of that patent document describes a measuring method of particle diameter as follows. “A molybdenum disulfide (MoS2) particle is presumed to be spherical and to have the same volume as an actual molybdenum disulfide particle. Therefore, the particle diameter of molybdenum disulfide in the present invention is a converted value calculated on a hypothesis that the particles are spherical. Based on the measured number and volume of particles, calculation can be made that these particles are spherical. Actually, the present inventors used a tester based on the electric resistance method (Colter O Counter, Multisizer type III, produced by Beckman Colter Corporation (aperture-tube 100 μm)) to measure the number and volume of particles and then determined the particle diameter. A desired range of average particle diameter can be provided by setting a desired diameter value of classified particles. A particle blasting treatment uses a molybdenum disulfide blasting material, in which the molybdenum disulfide particles are contained. The average particle diameter is an average value of such molybdenum disulfide particles.”
Patent Document 6: European Patent Publication 1236914-A1 (a patent family member of Japanese Unexamined Patent Publication (kokai) No. 2002-61652) discloses preparation of a coating liquid by using molybdenum disulfide, graphite, boron nitride or tungsten disulfide, preferably having an average particle diameter of 15 μm or less, more preferably 0.2 to 10 μm. An appropriate proportion of organic solvent (diluting agent) can be used to prepare a coating liquid, so as to dissolve or disperse these components in the coating liquid. The following descriptions are also provided. “The organic solvent adjusts viscosity to facilitate mixing. Usable organic solvent is not at all limited, provided that the thermosetting resin employed can be dissolved in the organic solvent. For example, when the thermosetting resin is polyamide-imide resin, it is possible to use xylene, N-methyl-2-pyrrolidone, toluene and the like in amounts of 100 to 300 mass parts relative to total 100 parts of these compounds.” Further, a method for forming a coating layer is disclosed as follows. “Rinsing with hot water and then drying with hot air are carried out. The above-mentioned coating liquid is diluted with an appropriate diluting agent and is then applied on the lining by spraying. Drying and sintering are carried out at 150-300 degrees C. When the post coating surface is rough, it can be smoothened by means of buffing or the like. A resin coating can be formed by roll transfer, tumbling, immersion, brush application, printing, or the like, other than spraying. A resin coating layer is preferably from 1 to 50 μm in thickness.” Patent Document 6 describes a method for measuring the solid lubricant's size but neglects shape of solid lubricant.
A coating layer described in Patent Document 7: United States Patent Publication U.S. 2004/0062660A1 (a patent family member of Japanese Unexamined Patent Publication (kokai) No. 2004-1139734) is formed by air-spraying the base resin and such a solid lubricant as molybdenum disulfide on the inner surface of a bearing. However, the publication does not describe the size of the solid lubricant particles.